1. Introduction
This invention relates to photoresists exhibiting decreased absorbency of light at activating wavelengths. More particularly, this invention relates to photoresists that utilize phenolic resin binders having lowered intrinsic optical absorbency.
2. Description of the Prior Art
Photoresists are photosensitive films used for transfer of an image to a substrate. They may be negative or positive acting. After a coating of a photoresist is formed on a substrate, the coating is selectively exposed through a photomask to a source of activating energy such as ultraviolet light. The photomask has areas that are opaque to activating radiation and other areas that are transparent to activating radiation. The pattern in the photomask of opaque and transparent areas define a desired image to be transferred to a substrate.
In the case of a negative photoresist, exposed portions of a photoresist coating become less soluble in a developer as the result of a photochemical reaction, thereby resulting in differential solubility between the exposed and unexposed portions. This difference in solubility allows for the selective removal of unexposed portions of the photoresist coating and the subsequent transfer of an image to a substrate.
In the case of a positive acting photoresist, exposed portions of the photoresist coating become more soluble in developer than unexposed portions as the result of a photochemical reaction allowing for selective removal of exposed areas by development.
Following development of a photoresist coating, portions of the substrate bared by development may be altered such as by etching. The historical background, types and processing of conventional photoresists are described by Deforest, Photoresist Materials and Processes, McGraw Hill Book Company, New York, Chapter 2, 1975 and by Moreau, Semiconductor Lithography, Principles, Practices and Materials, Plenum Press, New York, Chapters 2 and 4, 1988, both incorporated herein for their teaching of photoresist compositions and methods of making and using the same.
Most commercial photoresist formulations, both positive and negative, comprise a film forming binder and a light sensitive component. Many of these film forming binders are phenolic resins. For example, most positive acting photoresists currently in commercial use comprise a novolak resin and a naphthoquinone diazide sulfonic acid ester photoactive compound where the novolak resin is the reaction product of formaldehyde and a phenol. Examples of such photoresists are disclosed in U.S. Pat. Nos 4,377,631 and 4,404,272, incorporated herein by reference. Another class of positive acting photoresists comprise a polyvinyl phenol and a naphthoquinone diazide sulfonic acid ester. Examples of these photoresists are disclosed in U.S. Pat. Nos. 3,869,292 and 4,439,516, both incorporated herein by reference.
Many negative acting photoresists also utilize phenolic resins as the film-forming component of the photoresist. One such photoresist forms a negative image by image reversal. Photoresists capable of image reversal are positive acting photoresists, such as those described above, to which an image reversal agent, such as hexamethylolmelamine hexamethyl ether is added. Such a photoresist is disclosed in U.S. Pat. No. 4,581,321, incorporated herein by reference. Another negative acting photoresist employing a phenolic binder is disclosed in European patent application serial No. 0232972, incorporated herein by reference. The photoresist disclosed in this patent uses a photoacid generator sensitive to selective wavelengths of activating energy including deep U.V., a crosslinking agent, and a phenolic, acid hardening, polymeric binder. In the presence of activating radiation, the photoacid generator is cleaved generating a strongly acidic environment. Upon subsequent heating, the generated acid activates the crosslinking agent to form a base insoluble negative image in an image pattern.
An important property of a photoresist is image resolution. Whether the photoresist is positive or negative acting, a developed image of fine line definition having vertical side-walls is highly desired to permit transfer of the fine line image to an underlying substrate. Another important property of a photoresist is photospeed. Enhanced photospeed is especially important in applications where a number of exposures are needed, for example, in generating multiple patterns by a step and repeat process, or where activating energy of reduced intensity is employed. Increased photospeed also permits reduction in the light sensitive component of the photoresist and/or a decrease in the required energy of exposure for image formation.
Prior art photoresists have adequate line definition and photospeed for many commercial applications. However, the art is continuously seeking methods to improve each of these properties.